The 3GPP initiative “Licensed Assisted Access” (LAA) intends to allow Long Term Evolution (LTE) equipment to also operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum may have to be shared with other radios of similar or dissimilar wireless technologies, a so called listen-before-talk (LBT) method needs to be applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
In Europe, the LBT procedure is under the scope of EN 301.893 regulation. For LAA to operate in the 5 GHz spectrum, the LAA LBT procedure shall conform to requirements and minimum behaviors set forth in EN 301.893. However, additional system designs and steps are needed to ensure coexistence of Wi-Fi and LAA with EN 301.893 LBT procedures.
U.S. Pat. No. 8,774,209 B2, titled “Apparatus and method for spectrum sharing using listen-before-talk with quiet periods,” discloses a mechanism where LBT is adopted by frame-based orthogonal frequency-division multiplexing (OFDM) systems to determine whether the channel is free prior to transmission. A maximum transmission duration timer is used to limit the duration of a transmission burst, and it is followed by a quiet period. However, it is recognized herein that a fairer coexistence with other radio access technologies such as Wi-Fi is needed, while also satisfying EN 301.893 regulations.
LTE uses OFDM in the downlink and discrete Fourier transform (DFT)-spread OFDM, also referred to as single-carrier frequency-division multiple access (FDMA), in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid, as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of, for example, the control information. A downlink subframe with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
From LTE Rel-11 onwards, the above described resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10, only Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in FIG. 3 are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes. The PDCCH/EPDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands More specifically, the DCI includes downlink scheduling assignments, including PDSCH resource indication, transport format, hybrid-ARQ information and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the physical uplink control channel (PUCCH) used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments. The DCI also includes uplink scheduling grants, including PUSCH resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH. The DCI also includes power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.
One PDCCH/EPDCCH carries one DCI message containing one of the groups of information listed above. As multiple terminals can be scheduled simultaneously, and each terminal can be scheduled on both downlink and uplink simultaneously, it should be possible to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio-channel conditions, link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH, to match the radio-channel conditions.
In LTE, the uplink (UL) transmission scheduling command is transmitted from the eNB to the user equipment (UE). There is a fixed delay between the time the scheduling command is transmitted and the time the UE transmits the UL signal specified in the standard. This delay is provisioned to allow the UE time to decode the PDCCH/EPDCCH and prepare the UL signal for transmission. For a frequency division duplex (FDD) serving cell, this UL grant delay is 4 ms. For a time division duplex (TDD) serving cell, this UL grant can be greater than 4 ms.